The present invention relates generally to cooling of operating devices on spacecraft, and more particularly to a heat pipe structure with low expansion alloy heat pipes.
Orbiting spacecrafts carry various devices (e.g., electronic devices) that generate unwanted heat during operation of the spacecraft. Additionally, the spacecrafts are subjected to intense environmental conditions that exasperate problems caused by the unwanted heat. Typically, the unwanted heat is removed by cooling devices referred to as heat pipe assemblies. Heat pipe assemblies transfer heat by conduction to one or more heat pipes which then convects the removed heat to radiating panels. In certain heat pipe designs, the radiating panels distribute the heat across the panel to maintain a uniform temperature across the radiating panels to provide isothermal control. In other heat pipe designs, the radiating panels extend outside the orbiting spacecraft such that the heat is radiated into ambient space. Variable conductive heat pipes vary the amount of conductivity, so that the heat remains generally constant.
In practice, the heat generating device is affixed to or within a heat pipe assembly having a heat absorbing host structure equipped with radiating panels and having at least one heat pipe embedded therein. The heat from the heat generating device vaporizes a working fluid in the heat pipe which is then condensed and the heat of the condensation conducted to the radiating panels. The embedded heat pipe removes heat from the heat generating device at its evaporator end and the vapors are condensed at its condenser end. The heat pipe assemblies can be fabricated using similar material for the host structure and the heat pipe to avoid problems caused by components having a different coefficient of thermal expansion (CTE) that could cause stress failures in the assembly. Limiting material selection is a compromise that affects the efficiency performance of the heat pipe, contributes to the weight of the spacecraft and is less optimum in terms of heat removal.
Current spacecraft design requires the use of lightweight materials possessing near-zero CTE such as composites in order to meet the reduced weight, thermal management, and precision pointing requirements. Composite radiator panels with embedded aluminum heat pipes lead to thermally induced stresses due to the dissimilar CTE of the aluminum and composites facesheets. This causes detachment or debonding of the aluminum heat pipe from the composite panel and/or fracture of the composite facings leading to failure of the thermal control system. Therefore, it is desirable to limit the use of materials with dissimilar thermal expansion coefficients and at the same time meet the preferred thermal and structural requirements of spacecraft construction that lend themselves to easy assembly and fabrication.
Thermal performance efficiency of a heat pipe panel is determined by how effective the heat is transferred from the heat source to the heat pipe inner wall. For a conventional extruded aluminum heat pipe, the heat has to travel through the panel skin, across the skin/heat pipe interface, and through the heat pipe extrusion wall to the working fluid, which provides the cooling. This is an efficient design, however, countless efforts have been made to accommodate aluminum heat pipes in composite panels with minimal success. The CTE mismatch between the aluminum and the composite skin is large (e.g., greater than 10 times) and during normal operation, this CTE difference has caused unwanted joint failure and/or panel skin failure. Design sacrifices have been made to provide a more flexible composite panel to support the CTE mismatch of these materials.
Previous industry attempts have been made to construct a lightweight heat pipe radiator utilizing an organic matrix composite tube with an aluminum lined foil. However, the shortcomings to this approach are the poor through thermal conductance in the radial direction of the heat pipe that limits heat transfer, as well as the thermal stresses between the thin aluminum liner and the organic matrix composite tube. Prior art, such as xe2x80x9cThe Embedded Heat Pipe Structurexe2x80x9d, U.S. Pat. No. 6,065,529, assigned to TRW, Inc., provided a heat pipe assembly formed of dissimilar CTE materials for the heat pipe and the panel structure by inserting a thermally expandable fluid between the aluminum heat pipe and the composite panel structure. However, a major shortcoming to this arrangement includes a difficult, different, and cost ineffective assembly process to the conventional heat pipe assembly procedures.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to a heat pipe structure for facilitating temperature stability of heat generating devices residing in a spacecraft. The heat pipe structure comprises tubing made of a low thermal expansion alloy (e.g., Invar) and a low thermal expansion saddle (e.g., carbon-carbon) joined together and embedded into composite panel skins with minimal to no CTE mismatch. The saddle to composite facesheet interface can employ the common approach of using adhesive as the joining material. However, the saddle to heat pipe interface is joined together by using a higher conductivity joining media, such as tin-lead solder, which improves the thermal performance of the heat pipe assembly and minimize the temperature drop across the interface.
The heat pipes and the joining interfaces can be plated to facilitate the joining of the saddle to the heat pipes, for example, by soldering. The increase of the thermal heat transfer capability across the heat pipe/saddle interface optimizes the performance of the heat pipe assembly at the saddle/pipe interface. Typical interface media tends to possess low thermal conductivity, thus, lowering thermal performance or the ability to transfer heat from the heat source to the heat pipe. Soldering the saddle to the heat pipe provides a substantially higher thermal conductivity joining material with over a 50 times improvement in thermal transfer properties across this interface compared to the conventional adhesive bond approach. Additionally, the interface is stronger and more robust than conventional adhesive bonding.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.